The present invention is directed to the digital color imaging arts. It finds particular application to a system and method of printing a digital color image and will be described with particular reference thereto. Of course, it is to be appreciated that the invention is also applicable to printing monochrome images and to other environments and applications, such as other digital rendering systems, e.g., direct-to-plate systems, and video displays.
In commercial electronic printing, a printer, such as a graphics artist or document layout specialist, prepares a digital document or source file containing text, graphics, and images. The digital document is prepared in a PDL (page description language) or any other image data format on a host computer or any of various DTP (desk-top publishing) computers. Representative PDLs include PostScript (registered trademark of Adobe) and Interpress (registered trademark of Xerox).
The source file is transferred to an image processing device which interprets and processes the file for image formation. That is, the image processing device processes or decomposes the PDL file into a contone image of 8 bits per pixel or a byte map. The contone (continuous tone) image is an array of pixel information in a particular color space. One such color space is RGB which stands for the colors red, green and blue (RGB). Another color space is CMYK which stands for cyan (C), magenta (M), yellow (Y) and black (K), which are often the colors used in printing. In these examples, each pixel is represented as a combination of intensity values or grey scale pixel values, typically from 0 to 255, of each of the colors. The 0 value represents the maximum intensity of color and the 255 value represents the minimum intensity of color, i.e., white.
Still another color space used in digital image processing is luminance-chrominance space termed CIE 1976 L*a*b* (CIELAB). In L*a*b* space , L* is a lightness or luminance value, a* is a redness-greenness value, and b* is a yellowness-blueness value.
To print the image, the image processing device or a print engine includes a half-toner or screen generator which converts the contone image data in the particular color space into a raster image. Typically, the half toner renders a raster image for each of the print colors cyan, magenta, yellow and black (CMYK). Each raster image is composed of pixel data of 1 bit/pixel. Thus, each bit is simply an instruction whether or not to place a dot of color at a particular point on an output page.
The raster image data is then used to illuminate a photoreceptor for transferring the image onto a print sheet. In one example, a raster output scanner (ROS) exposes an electrophotographic surface such as a photoconductive belt or drum to record four latent images which correspond to the four raster images. One latent image is developed with cyan developer material. Another latent image is developed with magenta developer material, a third latent image is developed with yellow developer material, and a fourth latent image is developed a black developer material. These developed images are transferred to a print sheet in superimposed registration with one another to form a multicolored image on the print sheet. This multicolored image is then fused to the print sheet forming to form a color print of the document.
To image to the very edges of the print sheet, an oversized photoreceptor or photoconductive surface is used, i.e., a photoconductive surface is selected which is larger than the print sheet surface. This prevents unmarked areas at the edges of the print sheet if the sheet is misregistered relative to the photoconductive surface. Such an error is extremely noticeable for instance when an image containing a dark background is placed onto a white print sheet. This error shows up as a white rim on one or two edges of the print sheet.
The problem with an oversize photoconductive surface is that developer material is applied to regions of the photoconductive surface that will not contact the print sheet. This excess developer material must be cleaned off. Such a process overloads the cleaning system subjecting it to failure and/or additional cost and wastes developer material.
An alternative is to use an oversize photoconductive surface and an oversize print sheet. After the image is transferred to the print sheet, the edge of the print sheet is trimmed to the desired size. This technique also wastes developer material and adds the extra step of trimming to the printing process.
In the case of a digital reproduction machine or photocopier, a similar problem arises when a user reproduces a document that is undersized or incorrectly places a document onto the imaging platen off-center or at an off-angle. In such a case, the underside of the cover of the photocopier, which is often dirty, is imaged along with the document. This causes dirt, smudges, and the like to appear at in the edge regions of the reproduced document. Further, it causes an edge or edges of the original document to appear on the reproduced document.
To minimize the effect of this error, a digital image processing device in the photocopier blanks around the image. The conventional way of blanking around the image is by replacing margin pixels with a predetermined pixel value using a set size of the margin. Unfortunately, this usually results in digitally blanking out parts of the original document which might contain important data.
It would therefore be desirable to provide a printing system which images to the edge of a print sheet without excessive waste of developer material or overloading developer cleaning devices.
In accordance with one aspect of the present invention, a digital image printing apparatus is disclosed. The apparatus increases input pixel intensity values in an edge region of input image data to form output pixel intensity values. The output values are printed on an edge region of a print sheet. An edge processor reduces the input pixel intensity values to form output intensity pixel values. A printer prints the output intensity pixel values on a print sheet.
In a more limited aspect of the present invention, the edge processor accesses a look-up table memory which includes look-up tables for mapping the input intensity pixel values at an edge region of the input image to the output intensity pixel values.
In another aspect of the present invention, a method of digital image processing is disclosed. The method increases input pixel intensity values in an edge region of input image data to form output pixel intensity values that are printed on an edge region of a print sheet. The input pixel intensity values for the edge region of the input image are reduced to form output intensity pixel values. The output intensity pixel values are printed.
One advantage of the present invention is that it conserves developer applied to the edges of print sheets of full bleed images which saves printing costs.
Another advantage of the present invention is that reduces the burden on cleaning devices which remove excess developer material from photoconductive surfaces.
Yet another advantage is that it maintains the visibility of information at the edges of documents rather than blanking it out altogether.
Still yet another advantage is that it allows information, such as writing, at the edge of a page to be discerned.
Yet another advantage is that the width of the affected edge region of the page and the amount of image suppression in the edge region may be easily adjusted or programmed.
Still further advantages of the present invention will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description of the preferred embodiments.